Captain Pablo Mastroeni missed his second straight training session Thursday due to headaches. Considering Mastroeni’s recent history of concussion problems, it’s cause for serious concern for Pareja and his team.

“We want to be respectful of him; first he’s a person and I have much respect for Pablo,” Pareja told MLSsoccer.com on Thursday. “He’s still having headaches and the recommendation from the doctors is to keep exploring how we can help him.”

Mastroeni is believed to be suffering recurring headaches as a result of a knock to the head in the opening game of the season, a 2-0 victory against the Columbus Crew. The former US international was removed in the 74th minute for precautionary reasons.

He didn't travel with the team ahead of a game against the New York Red Bulls on Sunday (4 pm ET, ESPN).

“Pablo is still in recovery mode,” Pareja said. “We’re being very careful with him. [It’s] not just because he’s a big piece of our puzzle, but it’s a player that gives us everything every time.”

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Mastroeni’s recent history with concussions began last October, when a collision with Real Salt Lake’s Jámison Olave knocked the 35-year-old midfielder out for the rest of the season. During a preseason friendly in Hawaii last month, Mastroeni was removed in the first half after taking another blow to the head.

“We will do whatever it takes [to get Pablo better], because he is a human and his health is our first priority,” Pareja said.

The Rapids midfield will be in full-blown shuffle mode again New York, thanks to Jeff Larentowicz's suspension and Martín Rivero's ongoing attempts to receive his International Transfer Request form (ITC).

Those absences could bring Wells Thompson, a starter for most of 2010 and parts of 2011, and youngsters Ross LaBauex and Joseph Nane into the discussion.

“We are exploring,” Pareja said. “Training today was very good with the ideas that we have in mind. We still have a day and a half to figure it out, but I’m confident that the guys will do the job well.”

Center back Drew Moor also shifted forward into the midfield towards the end of practice, and said that he’s ready to fill in for Mastroeni if called upon.

“I’d be more than fine with doing it,” Moor told MLSsoccer.com. “[Pareja] put me in there for five or six minutes [during Thursday’s practice]. With some of the numbers we have missing right now, sometimes you have to make changes.”